1. Field of the Invention
This invention relates to a method for projection exposure applied to photolithography for preparation of semiconductor devices. More particularly, it relates to a method for exposure projection which allows for uniform resolution over an entire wafer surface.
2. Description of Related Art
In the field of semiconductor integrated circuits, submicron size processing has already been realized in mass-producing plants, while researches are currently conducted on half-micron size processing and even on quarter-micron size processing which is thought to be indispensable in 64 Mbit level DRAMs.
The technology which has played a key role in the progress of ultra-fine processing is photolithography. The progress so far achieved in ultra-fine processing owes much to reduction in exposure wavelengths of the exposure light and an increase in the numerical aperture (NA) of optical lenses of a stepper. However, the reduction in the exposure light wavelengths and the increase in the numerical aperture are not desirable from the viewpoint of increasing the depth of focus, because the depth of focus is proportionate to the wavelength of exposure light and inversely proportionate to the second power of numerical aperture.
On the other hand, the surface step difference of a semiconductor wafer as a member to be exposed tends to be increased recently with increase in the density of semiconductor integrated circuits. The reason therefore is that, for the sake of maintenance of circuit performance and reliability under the current tendency towards a three-dimensional device construction, reduction in the three-dimensional design rule is not progressing smoothly as compared to that in the two-dimensional design rule. If the surface of a semiconductor wafer presenting larger step differences is coated with a photoresist material to form a photoresist layer, larger step differences or fluctuations in film thicknesses are similarly produced in the so-formed photoresist layer. As to the step differences produced over finer patterns, the wafer surface was smoothed by a multilayer resist method. However, a step difference produced over a wider area, such as a step produced between memory cells and peripheral circuits, can not be compensated with the multilayer resist method. If such wafer is to be exposed to light, the position of an image plane can not but be selected to be a mid position between highs and lows of the step difference. In addition, the image plane can not be a completely flat plane due to distortion of the image plane of a projecting lens, while the wafer surface can not be completely normal to an optical axis of the projecting optical system.
Under these circumstances, and also as a result of increased light absorption of the photoresist material caused by use of shorter wavelengths, difficulties are raised in achieving uniform resolution on an entire wafer surface.
Thus the requirements for resolution and those for depth of focus are essentially contradictory to each other. For combatting this problem, researches are being conducted for developing techniques for achieving high contrast and hence high resolution through special artifices and methods for using an exposure device on the premise that the numerical aperture is suppressed to a predetermined level and a practically useful level of depth of focus is maintained.
Among these techniques is a so-called FLEX method, according to which, as disclosed in JP Patent KOKAI (Unexamined) Publication No. 58/17446 (1983), a number of light exposure operations is carried out while the image plane is shifted through the same photomask for effectively maintaining optical image contrast extended along the optical axis. For shifting the image plane, at least one of the photomask, semiconductor wafer or the projecting optical system is wobbled along the optical axis, as disclosed in the above mentioned Publication, or shifted stepwise or continuously each time light exposure is performed, as disclosed in JP Patent KOKAI Publication No. 63/64037 (1988).
The simplest FLEX method is a two-stage method having two different points on the optical axis as positions for the image plane. Referring to FIG. 1, an example in which the image plane is shifted along the optical axis (Z axis) by vertically driving a wafer-setting Z stage is hereinafter explained.
In this figure, the stage position (in .mu.m) along the Z axis is plotted on the horizontal axis. This stage position is defined as an offset (focal offset) from a reference point which is the position of the image plane set at a mean height position between the highs and lows of the step difference of the wafer (center focal position) as a point of origin. The direction proceeding towards a light source and that away from the light source are termed the minus (-) direction and the (+) direction, respectively, while the position of the image plane in which the wafer is shifted in the (-) direction and that in which the wafer is shifted in the (+) direction are termed the minus (-) focal position and the plus (+) focal position, respectively. The contrast in light intensities is plotted on the vertical axis. Two solid lines e and f represent contrasts in light intensities when projection light exposure is performed with the same volume of light exposure with the image plane having been shifted -1.0 .mu.m and +1.0 .mu.m from the point of origin, respectively. A broken line g represents contrasts of light intensities obtained by synthesizing the curves e and f. Although the light intensity contrast strictly is not in linearly correlated with the volume of light exposure, it is assumed herein to correspond approximately to the volume of light exposure for convenience.
In order for the pattern to be dissolved satisfactorily on the entire surface of the wafer, a light intensity contrast higher than a predetermined level need to be maintained within a predetermined extent along the optical axis as determined by the step difference of a wafer surface, distortion of an imaging surface of a projecting lens, tilt of the wafer or the like. If it is assumed that an appropriate extent of the light intensity contrast corresponding to a practically sufficient resolution is 0.5 to 0.8 and the range along the Z axis with which such range of the light intensity contrast is achieved is defined as a focal margin, the focal margin as viewed on the synthesized light intensity contrast shown by the broken line g is approximately 1.2 .mu.m along the (-) and (+) directions, or 2.4 .mu.m in sum, as shown by ranges B.sub.1 and B.sub.2. However, the synthesized light intensity contrast becomes lower than 0.5 in the vicinity of the center focul position so that stable resolution can not be achieved. The result is that inconveniences such as fluctuations in contact hole diameters, for example, are produced.
On the other hand, a three-stage method has also been proposed, in which light exposure at the center focal position is additionally performed in the above mentioned two-stage method. This alternative method is hereinafter explained by referring to FIG. 2.
In an example, shown in FIG. 2, light exposure is performed not only at the focal offsets at .+-.1.0 .mu.m, but also at the center focal position. Thus, in FIG. 2, three solid lines h, i and j represent light intensity contrast curves when projection light exposure is performed with the same volume of light exposure with the focal offsets of -1.0 .mu.m, 0 .mu.m and +1.0 .mu.m, respectively. A broken line k represents a light intensity contrast curve obtained by synthesis of three curves h, i and j. It is noted that, since the total volume of light exposure is the same as that with the above mentioned two-stage method, the volume of light exposure for one light exposure operation is less than that in the case of the two-stage method.
With this technique, the focal margin as seen on the synthesized light intensity contrast curve shown by the broken line k is obtained as a continuous range which, as shown by a range C in FIG. 2, in contrast to the two-stage method, meaning that instability in the vicinity of the center focal position has now been eliminated.
However, the focus margin with the above mentioned three stage method is about 2.2 .mu.m, as shown in FIG. 2, which is less than that obtained with the two-stage method shown in FIG. 1. Thus the merit proper to the three-stage method may not be said to be functioning satisfactorily.